Fix: Use credentials from FTP URIs

[theworker02]

Resolved / Related Issues

• Closes Feature: Use username and password from the FTP address instead of showing dialog #14953

Steps used to test these changes

1. Ran dotnet build src/Files.App.Storage/Files.App.Storage.csproj --configuration Debug --arch x64 -v:quiet -clp:ErrorsOnly.
2. Verified that FTP URIs with user info now create a NetworkCredential from the decoded URI username and password before falling back to saved or anonymous credentials.

Manual FTP-server verification was not performed in this environment.

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[hez2010]

LGTM. Thanks!

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